(1) Field of the Invention
The present invention relates to a voice coding/decoding system having a transmitting part for transmitting a coded signal of an input voice signal and a receiving part for receiving and decoding the coded signal transmitted from the transmission part, and more particularly, to a voice coding/decoding system that does not utilize a buffer memory.
In general, the transmission part has a coder for adaptive differential PCM (ADPCM), adaptive transform coding (ATC) and so forth, and an entropy coder. The coder provides an encoded word which is obtained by compacting the amount of input information. The coded word is then changed to an entropy-coded word by the entropy coder. The entropy-coded word is transmitted from the transmission part to the receiving part. The receiving part conversely decodes the entropy-coded word to reproduce the voice signal.
Accompanied by the developments of digital lines in recent years, a system for compacting the amount of information to obtain coded signals at a high efficiency has been desired in order to effectively utilize the lines.
Also, to store voice information in a voice storage or voice response system, it is important to decrease the capacity of the storing memory. The system for compacting the amount of information and for coding at a high efficiency will satisfy the above requirements.
(2) Description of the Related Art
As a voice signal coding apparatus, a coding transmission apparatus is conventionally known in which a coder and an entropy coder are combined to code voice signals with high efficiency. The coder carries out compacting and coding of voice signals by means of adaptive differential PCM (ADPCM), adaptive transform coding (ATC) and so forth. The entropy coder carries out removing of statistical redundancy in quantized values.
In the coding transmission apparatus for carrying out coding of voice signals at high efficiency, since the voice signals have nonuniform probability of symbols, the statistical characteristics of the output of the coder are changed so that the code lengths, i.e., bit rates, of the entropy-coded results are not constant, and the bit rate may happen to be larger than the allowable transmission bit rate so that the transmission becomes impossible.
Conventionally, to solve the above problem, the entropy encoded result is stored in a buffer memory and is transmitted from the buffer memory through the transmission line. To this end, a buffer control system has been employed in which the entropy-coded results are controlled by changing the quantization characteristics of a quantizer in the coder. This conventional system is described in "On the Information Rate Control in Entropy-coded Speech Transmission Systems", written by M. Copperi, CSELT Rapportitecnici Vol. X-No. 6-DECEMBER, 1982 PP 435-449.
In the above described conventional art, a buffer memory is used to control the entropy coded results. In practice, however, an overflow or an underflow of the buffer memory occurs depending on the characteristics of the coder and the entropy coder.
Further, in the control method by means of the buffer memory, the control is carried out in such a way that the input/output characteristics of the quantizer in the coder is changed so that the statistical characteristics of the quantized output codewords are controlled so as to make the entropies small. Namely, the entropies concentrate to certain values. Therefore, the characteristics of the quantizer itself do not always become optimum, and the characteristic of the coder is not always good.
Further, when there is a transmission error, the error is transmitted in the entropy decoder. Therefore, once a transmission error is generated, the error is superimposed on the next decoded result causing a problem such that the one transmission error influences several samples.